1. Field of the Invention
This invention relates to determining, from a telephone central office, whether or not a subscriber telephone is in a receiver-off-hook condition.
2. Description of the Prior Art
A frequent telephone problem is where a first subscriber reports repeated busy signals when trying to reach a second subscriber. Assuming the first subscriber has dialed correctly, busy signals may be produced by a leakage path between the second subscriber's telephone line conductors or by the second subscriber's telephone being in a receiver-off-hook-and-in-use condition or in a receiver-off-hook-and-not-in-use condition.
Heretofore, it has been a common practice to identify the cause of the problem by an operator or craftsperson gaining access to the line in question and identifying the cause by what is heard on the line. This testing procedure requires what is considered to be an unreasonable length of time for testing, possible ambiguity with respect to the results produced and, furthermore, possible interference with a customer's conversation.
To improve telephone service, consideration has been given to automating this entire test procedure. With this in mind, the present inventor invented a receiver-off-hook-and-in-use detector which forms the subject matter of his copending application Ser. No. 700,293, filed on even date herewith. When this detector has determined that a receiver is not off-hook-and-in-use, tests must still be performed to determine if the problem is caused by either a receiver-off-hook-and-not-in-use condition or by a line leakage condition.
The two remaining conditions may be distinguished from one another by taking advantage of the nonlinear resistance versus voltage characteristic of a receiver-off-hook telephone and the substantially linear resistance versus voltage characteristic of a conductor-to-conductor leakage path. In particular, a plurality of dc impedance measurements, where each measurement is made at a higher voltage level than the prior measurement, are performed on the line. Substantially identical measurements indicate a conductor-to-conductor leakage path while unlike measurements indicate a receiver-off-hook condition.
Two shortcomings exist in the last-mentioned test. First, central office low resistance equipment and battery must be disconnected from the line under test. This requires the services of a technician. Secondly, the required testing time is much longer than, and therefore not commensurate with, the testing time required by the present inventor's previously mentioned receiver-off-hook-and-in-use detector.